Sandbox Reserved 471: Difference between revisions
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[[Image:Actin Image.gif]] | [[Image:Actin Image.gif]] | ||
Actin exists in both its monomeric (globular, G-actin) and polymeric (filamentous or F-actin) form. The actin monomer is approximately pear shaped. Actin is composed of 376 residues that are folded into two large domains, each comprised of two subdomains. Its secondary structure is 41% helical (22 helices; 153 residues) and 20% beta sheet (20 strands; 77 residues). The actin filament is a double-stranded, right-handed helix with a half-pitch of 37 nm and a one-start left-handed genetic helix with a rise of 2.75 nm per monomer. The width of the filament is within the range of 7–10 nm. <ref>PMID: 20672362</ref> The large domains are organized to form a hinged molecule with a deep cleft. Subdomains 1 and 3 are structurally related, whereas subdomains 2 and 4 can be viewed as large insertions into subdomains 1 and 3, respectively. There is moderately little contact between the two major domains of actin; the polypeptide chain passes twice between these domains at the loop centered at residue Lys336 and at the linker helix Gln137-Ser145, which functions as the hinge axis between the domains. As a result, two clefts are formed between the domains. Within the upper cleft are the cofactors, an adenine nucleotide and a divalent metal ion, usually magnesium. It is thought that the cofactors interact with the domains on either side, increasing their connectivity. The lower cleft between domains 1 and 3 is lined by residues Tyr143, Ala144, Gly146, Thr148, Gly168, Ile341, Ile345, Leu346, Leu349, Thr351, and Met355, which are predominantly hydrophobic. The hydrophilic residues can also be seen, however they are not as involved in the binding site. This cleft constitutes the major binding site for most ABPs, and is thus called the targetbinding or hydrophobic cleft.Most of the interaction that occur in the subunit are electrostatic in character but there are also hydrophobic interactions.<ref>PMID: 2395459</ref> | Actin exists in both its monomeric (globular, G-actin) and polymeric (filamentous or F-actin) form. The actin monomer is approximately pear shaped. Actin is composed of 376 residues that are folded into two large domains, each comprised of two subdomains. Its TextToBeDisplayed:secondary structure is 41% helical (22 helices; 153 residues) and 20% beta sheet (20 strands; 77 residues). The actin filament is a double-stranded, right-handed helix with a half-pitch of 37 nm and a one-start left-handed genetic helix with a rise of 2.75 nm per monomer. The width of the filament is within the range of 7–10 nm. <ref>PMID: 20672362</ref> The large domains are organized to form a hinged molecule with a deep cleft. Subdomains 1 and 3 are structurally related, whereas subdomains 2 and 4 can be viewed as large insertions into subdomains 1 and 3, respectively. There is moderately little contact between the two major domains of actin; the polypeptide chain passes twice between these domains at the loop centered at residue Lys336 and at the linker helix Gln137-Ser145, which functions as the hinge axis between the domains. As a result, two clefts are formed between the domains. Within the upper cleft are the cofactors, an adenine nucleotide and a divalent metal ion, usually magnesium. It is thought that the cofactors interact with the domains on either side, increasing their connectivity. The lower cleft between domains 1 and 3 is lined by residues Tyr143, Ala144, Gly146, Thr148, Gly168, Ile341, Ile345, Leu346, Leu349, Thr351, and Met355, which are predominantly hydrophobic. The hydrophilic residues can also be seen, however they are not as involved in the binding site. This cleft constitutes the major binding site for most ABPs, and is thus called the targetbinding or hydrophobic cleft.Most of the interaction that occur in the subunit are electrostatic in character but there are also hydrophobic interactions.<ref>PMID: 2395459</ref> | ||
Under physiologic conditions, G-actin is transformed to F-actin by ATP. ATP is one of the most common ligands binding in the cleft of actin. After G-actin is bound to an ATP molecule it can then bind to another ATP bound monomer to form an unstable dimer. It can then add a third ATP bound monomer to create a stable trimer that serves as a basis for building fibrous actin (F-actin). Once the F-actin is formed the ATP can be hydrolyzed, keeping the bound ADP and releasing an inorganic phosphate. The polymer can undergo a process known as treadmilling. This is where the polymer continuously grows at its positive end but disassembles at its negative end. This process requires a permanent source of energy such as ATP <ref>Boron, W., & Boulpaep, E. (2008). Medical physiology. (2 ed., pp. 14-20). Saunders Elsevier. Retrieved from http://books.google.com/books/about/Medical_Physiology_E_Book.html?id=HlMJRw08ihgC</ref> | Under physiologic conditions, G-actin is transformed to F-actin by ATP. ATP is one of the most common ligands binding in the cleft of actin. After G-actin is bound to an ATP molecule it can then bind to another ATP bound monomer to form an unstable dimer. It can then add a third ATP bound monomer to create a stable trimer that serves as a basis for building fibrous actin (F-actin). Once the F-actin is formed the ATP can be hydrolyzed, keeping the bound ADP and releasing an inorganic phosphate. The polymer can undergo a process known as treadmilling. This is where the polymer continuously grows at its positive end but disassembles at its negative end. This process requires a permanent source of energy such as ATP <ref>Boron, W., & Boulpaep, E. (2008). Medical physiology. (2 ed., pp. 14-20). Saunders Elsevier. Retrieved from http://books.google.com/books/about/Medical_Physiology_E_Book.html?id=HlMJRw08ihgC</ref> | ||
